This invention relates to stents. In general, this invention relates to stents made more flexible, based on their overall structure; specifically, this invention relates to varying the wall thickness of a stent in order to create a more flexible stent; and also to improve radioopacity of the stent while not effecting its flexibility.
A stent is commonly used as a tubular structure left inside the lumen of a vessel or duct of the body, in order to relieve an obstruction in the lumen or is duct. Commonly, stents are inserted into the lumen in their unexpanded form; the stents are then expanded on their own (or with the aid of a second device) in situ at the obstruction. A typical method of expansion occurs through the use of a catheter-mounted stent on an angioplasty balloon, which balloon is inflated within the stenosed vessel or body passageway. This inflation causes the obstructions of the vessel associated with the wall components of the vessel to compress. Thereafter, the stent holds the obstruction in place at the vessel wall, and an enlarged lumen is obtained.
It is fairly commonly believed that usage of a stents is a technology transforming the type of therapy used against stenosis of a lumen. That is, in the absence of a using a stent, (in other words, when a balloon used alone) restenosis often occurs, as a result of elastic recoil of the stenotic lesion away from the vessel wall. So, stents have been hailed as a breakthrough in the use of science to improve medicinal therapies.
Although a number of stent designs have been created, the designs of these stents have exhibited a number of limitations, such as a general restriction on the dimension of the stent. For instance, typical stents have been limited to a pair of generally rigid ends (approx. 8 mm) and a flexible middle section (anywhere from about 7 mm to about 21 mm). This device is formed of multiple parts and is not continuously flexible along the longitudinal At axis. Other stent designs with rigid segments and flexible segments have also been described.
Other stents are described as longitudinally flexible, but consist of a plurality of cylindrical elements connected by flexible members. This design has at least one important disadvantage, for example, according to this design, protruding edges occur when the stent is flexed around a curve raising the possibility of inadvertent retention of the stent on plaque deposited on arterial walls. This may cause the stent to embolize or more out of position and further cause damage to the interior lining of healthy vessels. Thus, balloon expandable stents known in the art, generally compromise axial flexibility to permit expansion and provide overall structural integrity.
However, it is further more desirable to be able to vary the flexibility of the stent along its length. In other words, it may be more desirable to have a stiffer stent at the proximal end and a more flexible stent at the distal end, or vice versa. To be able to vary the flexibility of the stent along its length could provide for emplacement of the proximal end in a narrower or more tortuous coronary artery, while stabilizing the stent at its proximal end. Or, it might be desirable to vary the flexibility in the center of the stent in order to achieve a bend of a particular coronary artery. If the stent could be produced reliably and quickly, it may be possible to examine the patient, determine the degree of tortuousity of the patient""s coronary arteries, and then construct the stent to tailor to the patient.
The present invention overcomes some perceived shortcomings of prior art stents by providing a stent with axial flexibility. In a preferred embodiment, the stent has a first end and a second end with an intermediate section between the two ends. The stent further has a longitudinal axis and comprises a plurality of longitudinally disposed bands, wherein each band defines a generally continuous wave along a line segment parallel to the longitudinal axis. A plurality of links maintains the bands in a tubular structure. In a further embodiment of the invention, each longitudinally disposed band of the stent is connected, at a plurality of periodic locations, by a short circumferential link to an adjacent band. The wave associated with each of the bands has approximately the same fundamental spatial frequency in the intermediate section, and the bands are so disposed that the waves associated with them are spatially aligned so as to be generally in phase with one another. The spatially aligned bands are connected, at a plurality of periodic locations, by a short circumferential link to an adjacent band. In particular, at each one of a first group of common axial positions, there is a circumferential link between each of a first set of adjacent pairs of bands.
At each one of a second group of common axial positions, there is a circumferential link between each of a second set of adjacent rows of bands. Thus along the longitudinal axis, a common axial position occurs alternately in the first group and in the second group of axial positions. Then, the first and second sets are selected so that a given band is linked to a neighboring band at only one of the first and second groups of common axial positions.
In a preferred embodiment of the invention, the spatial frequency of the wave associated with each of the bands is decreased in a first end region lying proximate to the first end and in a second end region lying proximate to the second end. By comparison this will make the ends of the stent xe2x80x9cstiffer.xe2x80x9d In a further embodiment of the invention, the spatial frequency of the bands in the first and second end regions is decreased by 20% compared with the spatial frequency of the bands in the intermediate section. The first end region may be located between the first end and a set of circumferential links lying closest to the first end and the second end region lies between the second end and a set of circumferential links lying closest to the second end. The widths of corresponding sections of the bands in these end regions, measured in a circumferential direction, are greater in the first and second end regions than in the intermediate section. Each band includes a terminus at each of the first and second ends and the adjacent pairs of bands are joined at their termini to form a closed loop.
In a further embodiment of the invention, a stent is provided that has first and second ends with an intermediate section between the two. This stent has further increased axial flexibility. This stent includes a plurality of longitudinally disposed bands, wherein each band defines a generally continuous wave having a spatial frequency along a line segment parallel to the longitudinal axis, the spatial frequency of the wave associated with each of the bands being decreased in a first end region lying proximate to the first end and in a second end region lying proximate to the second end, in comparison to the spatial frequency of the wave in the intermediate section; and a plurality of links for maintaining the bands in a tubular structure. The first and second regions have been further defined as the region that lies between the first and second ends and a set of circumferential links lying closest to the first end and second end. In a further embodiment the widths of the sectionals of the bands, measured in a circumferential direction, are greater in the first and second end regions than in the intermediate section.
In yet an additional embodiment, the stent is divided into a group of segments. Each of the segments are connected by a flexible connector. In addition, the stent segments are provided with enhanced flexibility at the flexible connectors, due to the geometrical configuration of the flexible connectors.
Furthermore the objects of the invention are accomplished with the stent being made flexible along its length. The stent initially starts as a metal sheet. It is rolled in its central region to a specified wall thickness. Thereafter, the stent is photochemically etched to produce the desired cell pattern of the design of the stent. Then, the stent is folded and the metal is joined to give rise to a stent with multiple wall thickness. For instance, larger wall thicknesses at the end of the stent versus smaller wall thicknesses at its center. Various other possible manufacturing methods are certainly perceived to be possible with respect to this stent. Alternately, the stent can be etched in its tubular configuration with the same desired effect.
These and other objects of the present invention will be better understood with reference to the attached figures and Detailed Description of the Invention which follow.